Dark stool is usually caused by something you ate, drank, or took as a supplement. But in some cases, it signals bleeding in the upper digestive tract, which needs prompt medical attention. The key difference comes down to texture, stickiness, and whether you have other symptoms.
Foods That Turn Stool Dark
Several common foods can make your stool look dark brown, dark green, or even black. Blueberries are one of the most common culprits, especially if you eat a large amount. The deep pigments pass through your digestive system and can darken stool enough to look almost black. Black licorice does the same thing. Beets contain a red pigment called betanin that can give stool a deep reddish or dark appearance that some people mistake for blood. Artificial food dyes, particularly from brightly colored candy or frosting, can also mix together in your gut and produce a surprisingly dark result.
Because food takes about three days on average to move through your digestive system, you may not connect what you ate with the color change. There can also be a several-day delay after you stop eating the food before your stool returns to its normal brown color. If you recently ate any of these foods, that’s likely your answer.
Medications and Supplements
Iron supplements are one of the most frequent causes of dark or black-looking stool. The effect is dose-dependent: in one study of pregnant women, 8% of those taking a lower dose of iron experienced black stools, compared to 31% of those on a higher dose. If you recently started an iron supplement and noticed the change, the two are almost certainly related.
Pepto-Bismol (bismuth subsalicylate) is another well-known cause. When the active ingredient, bismuth, contacts small amounts of sulfur naturally present in your saliva and digestive system, it forms a black compound called bismuth sulfide. This substance darkens your stool as it passes through. The effect is harmless and clears up within a few days of stopping the medication.
When Dark Stool Means Bleeding
The medical term for black, tarry stool caused by bleeding is melena. It happens when blood from the upper digestive tract (the esophagus, stomach, or upper small intestine) is broken down by stomach acid as it travels through the gut. This chemical process turns the blood black before it exits your body.
Melena looks and feels different from stool that’s dark because of food. It has a distinctly tarry, sticky texture that patients often remember. It also has an unusually strong, foul smell. If your stool is black but formed normally and doesn’t have that tar-like consistency, bleeding is less likely.
Common causes of upper digestive bleeding include stomach ulcers, inflammation of the stomach lining, and tears in the esophagus. In people with liver disease, increased pressure in the veins around the esophagus and stomach can cause swollen blood vessels called varices. These varices can rupture and bleed, sometimes severely. Portal hypertensive changes in the gut lining have been reported in 20% to 98% of patients with liver cirrhosis, though acute bleeding from this is relatively uncommon.
Warning Signs That Need Immediate Attention
If your dark stool is accompanied by any of the following, go to an emergency room or urgent care:
- Vomiting blood or vomit that looks like coffee grounds
- Dizziness, weakness, or lightheadedness, which can indicate significant blood loss
- Heart palpitations or shortness of breath
- Several consecutive days of black, tarry stool
These symptoms suggest active bleeding that may be substantial enough to affect your blood pressure and oxygen delivery. Even without these red flags, black tarry stool that you can’t explain with food, iron, or bismuth warrants a call to your doctor.
How Doctors Check for Hidden Blood
If your doctor suspects bleeding but isn’t sure, they may order a stool test that detects hidden (occult) blood. The immunochemical version of this test is highly accurate for colorectal cancer specifically, with 96% sensitivity, meaning it catches nearly all cases. For other types of lesions in the lower GI tract, the test is less sensitive at about 24%, but its specificity is close to 90%. This means a positive result is meaningful, but a negative result doesn’t completely rule out all sources of bleeding. If suspicion remains high, your doctor may recommend an endoscopy to look directly at the lining of your upper digestive tract.
Dark Stool in Newborns
If you’re a new parent, very dark stool in the first few days of life is completely normal. Newborns pass meconium, a thick, dark green-to-black substance, within the first 24 to 48 hours after birth. Over the next few days, stool transitions from dark green to the yellow, seedy appearance typical of breastfed infants. This transition usually completes by day three to five.
A newborn who hasn’t had a bowel movement within the first 24 hours, or one who develops vomiting and a swollen belly in the first few days, needs to be seen by a healthcare provider right away. After that initial meconium phase, persistently dark stool in an infant is not typical and should be evaluated.
A Simple Way to Sort It Out
Start by thinking about what you’ve consumed in the past three to five days. Iron supplements, Pepto-Bismol, blueberries, black licorice, and dark-colored foods are the most common harmless explanations. If you can identify one of these, try stopping it and waiting a few days to see if your stool returns to normal.
If you can’t identify a dietary or medication cause, pay attention to the stool’s texture. Normal stool that happens to be dark will still look formed and relatively familiar. Melena is sticky, tar-like, and has a particularly strong odor. Combined with any symptoms of blood loss (feeling faint, rapid heartbeat, vomiting), that pattern points toward bleeding that needs medical evaluation without delay.